1. Field of the Invention
This invention is directed to electromagnetic work coils for electromagnetic force machines that produce a pulling (tension) force on a conductive material, which may be used for metal forming, removing dents, or performing nondestructive proof load tests.
2. Description of the Related Art
In the past, a variety of electromagnetic force (EMF) pulling machines and electromagnetic coils have been developed for use in the production and maintenance of conductive panel work pieces to perform nondestructive tests on panel bonds and to remove dents.
U.S. Pat. No. 4,148,091, issued to Karl A. Hansen et al. on Apr. 3, 1979 entitled “Electromagnetic force machine with universal portable power supply,” and U.S. Pat. No. 3,825,819, issued to Karl A. Hansen et al on Jul. 23, 1974 entitled “Dynamic Proof Loading of Metal Bond Structures Using Pulsed Magnetic Fields,” describe such a machine. U.S. Pat. No. 5,046,345, issued to Peter B. Zieve on Sep. 10, 1991 entitled “Power Supply for Electromagnetic Proof Load Tester and Dent Remover,” describes a power supply for such a machine. U.S. Pat. No. 4,986,102, issued to I. Glen Hendrickson et al. on Jan. 22, 1991 entitled “Electromagnetic dent remover with tapped work coil,” describes another power supply and electromagnetic coil for such a machine.
U.S. Pat. No. 4,061,007, issued to Karl A. Hansen et al. on Dec. 6, 1977 entitled “Electromagnetic dent remover with electromagnetic localized work coil,” and U.S. Pat. No. 4,127,933, issued to Karl A. Hansen et al. on Dec. 5, 1978 entitled “Method of making work coil for an electromagnetic dent remover,” describe several electromagnetic coils for use with such a machine and the methods used to manufacture them. U.S. Pat. No. 4,116,031, issued to Karl A. Hansen et al. on Sep. 26, 1978 entitled “Flux concentrator for electromagnetic pulling,” describes another electromagnetic coil for such a machine that utilizes a secondary coil and is referred to as a flux concentrator.
U.S. Pat. No. 5,575,165, issued to Thomas J. Roseberry on Nov. 19, 1996 entitled “Method of dent removal using a resonance damping vacuum blanket,” describes a vacuum system for stiffening the panel work piece for use with such a machine and electromagnetic coil.
The electromagnetic work coils for electromagnetic force (EMF) pulling machines create a pulling (tension) force on conductive material by first presenting a slowly increasing tangential magnetic field that penetrates into a conductive panel work piece. It is a requirement that the magnetic field be presented slowly enough so the skin depth of eddy currents reacting in the work piece is greater than the material thickness. This minimizes the reacting Lorentz force so that the work coil and conductive panel work piece do not push away from each other. Next, the work coil quickly must collapse the magnetic field from the face of the work piece. The collapse must be sufficiently rapid enough such that the skin depth of reacting eddy currents is less than the panel work piece thickness. The reacting eddy currents in the presence of the magnetic field cause a Lorentz force that is oriented normal to both and draws the work piece and the work coil together. This pulling force, or tension force, may be used for nondestructive pulling tests or to pull a dent out of the work piece.
The physical requirements of the work coil for an EMF pulling machine are different than a coil or device used with an EMF machine that pushes. In a typical EMF machine that pushes, a repulsive force is used to compact, swage, or rapidly move a conductive part away from the work coil. Since the force on the work coil is in the opposite direction, the design for a pushing work coil is significantly different. The coil windings can be backed and supported directly to withstand the pushing forces. For the pulling work coil the windings must withstand a pulling force. The windings must slowly present the magnetic field with a significant current density. This results in a greater amount of energy dissipated in the work coil in the form of heat that must be dissipated to maintain conductance and material strength.
While prior art work coils have proven to be satisfactory for some work, improvements are desired. It is desired to pull dents from thicker, harder, or less conductive materials like titanium. It is further desired to pull higher aspect ratio dents with greater plastic deformation. This requires a coil that can withstand greater thermal and mechanical stress. As the panel work piece material increases in thickness, the presentation of the magnetic field has to be slower to prevent pushing the panel work piece and work coil away from each other. This requires increased energy from the EMF power supply that will be converted to heat in the electrical resistance of the coil windings. The work coil must be cooled or capable of dissipating the extra heat. Additionally, for a thicker panel work piece, it is desirable to increase the magnetic field strength to create higher pressure necessary to perform work on the material. The increased magnetic field requires an increased current density and will impart additional mechanical and thermal stress on the coil windings. We desire to do work to the work piece without doing work on the coil so that the coil can be reused and jobs accomplished safely. At the energy levels of interest, some prior art coils have failed destructively ejecting molten copper.
As the panel work piece thickness is increased, a repulsive force increases due to the imperfect slow field presentation that will reduce the effectiveness of the process. This force may be minimized by slowing down the field presentation or it can be counteracted by preloading or pushing the work coil into the work piece. If the preloaded force is overcome, the gun and panel work piece separation velocity can be minimized with a heavier gun and more rigidly backed panel work piece.
Another method developed by the Inventor to improve performance with thicker panels is to apply a vacuum between the work coil and the panel work piece. This is attractive because it provides atmospheric pressure applied evenly and directly behind the panel material and behind the work coil. This technique should not be confused with the concept taught in U.S. Pat. No. 5,575,165 where a vacuum blanket is used to stiffen a thin aluminum panel around the area to be pulled to dampen any resonance. The performance of preloading is an improvement in some cases whether it is applied by the operator pushing or by an applied vacuum.